Can anyone help with bra sizing?

Currently I am wearing a 34C. My breast spill out when I bend over. When I lift my arms, the under wire rises and does not fall flat on my ribcage. I know that this is a sign that the bra is too small. I have gone to department stores and to "bra specialists" at stores such as Victoria's Secret and Fredericks to size me and the size range has gone from a 34B-36DD. I have followed many website instructions but seem to always get the measurement of a 34A. I am very frustrated with this and have not had a proper fitting bra for years. Before I shell out a bunch of money on a bra that doesn't fit, I was wondering if any of you out there in the yahoo answers community know how to properly size themselves or others for a well fitting bra, or at least show me a way that I can accurately measure it myself!